1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system wherein chromatic dispersion compensation and monitoring control thereof are executed in a photonic network having a node device which performs an optically branched insertion and an optical path shifting of an optical signal as is.
2. Description of the Related Art
Conventionally, construction of a photonic network having topology such as a ring and a mesh which utilize an OADM (Optical Add/Drop Multiplexing) node for performing the optically branched insertion of an optical signal as is, without performing photoelectric conversion, and a HUB node for performing the optical path shifting in wavelength unit (including drop and continue) is required. Realization of flexible and simple operation and maintenance is expected in a photonic network such as this.
The optical wavelength of each signal is regarded as the unit of a path in the photonic network to which a WDM (Wavelength Division Multiplexing) Optical Transmission Technology is applied. This path is called a wavelength path. In addition, the branched insertion or optical path shifting of an optical signal is performed by this wavelength path unit, in the photonic network to which the OADM node which performs the optically branched insertion of the optical signal as is, without performing photoelectric conversion, or the HUB node which performs the optical path shifting is applied.
Here, attention is focused particularly on the photonic network to which the HUB node, which performs the optical path shifting of the optical signal as is, is applied, in the specification according to the present invention.
FIG. 1 is a diagram showing an example of the photonic network to which the HUB node, which performs the optical path shifting in the optical signal as is, is applied.
The network shown in FIG. 1 comprises a transmitting end (the transmitting end A and the transmitting end B in the figure), an optical amplification repeater, a transmission line which comprises optical fibers (Link A, B and C in the figure), a HUB node, and a receiving end.
Each optical amplification repeater comprises single or multistage optical amplifiers and a dispersion compensation module (DCM in the figure), which is disposed between either before or after the single optical amplifier or between the stages of multistage optical amplifiers to compensate the accumulated chromatic dispersion of the optical fiber as a transmission line. Each transmitting end comprises optical transmitters of each wavelength (shown as OS in the Figure), an optical multiplexer (shown as MUX in the Figure), single or multistage optical amplifiers, and the dispersion compensation module which is disposed either before or after the single optical amplifier or between the stages of multistage optical amplifiers. The HUB node has a plurality of input ports and output ports and comprises single or multistage optical amplifiers which is disposed at each I/O port, the dispersion compensation module which is disposed either before or after the single optical amplifier or between the stages of multistage optical amplifiers, a wavelength selective switch (λ-switch in the figure) which switches optical paths according to each wavelength, a variable dispersion compensator (VDC in the figure), a control circuit of the variable dispersion compensator (Cont. in the figure) and a chromatic dispersion monitor (Disp. Mon. in the figure) The optical receiving end comprises single or multistage optical amplifiers, and the dispersion compensation module which is disposed either before or after the single optical amplifier or between the stages of multistage optical amplifiers, the variable dispersion compensator (VDC in the figure), the control circuit of the variable dispersion compensator (Cont. in the figure) and the chromatic dispersion monitor (Disp. Mon. in the figure). In addition, the variable dispersion compensator is capable of compensating the accumulated chromatic dispersion and the dispersion slope (primary wavelength dependent characteristics of accumulated chromatic dispersion).
In the optical transmitting end A and the optical receiving end B, optical signals output from optical transmitters of each wavelength are input to the transmission paths after they are wavelength multiplexed by optical multiplexers, optically amplified by optical amplifiers as optical signals, and chromatic dispersion compensated by the dispersion compensation module. A WDM signal output from the optical transmitting end A transmits Link A, and a WDM signal outputted from the optical transmitting end B transmits Link B. Each WDM signal is connected with the HUB node.
The WDM signal input to the HUB node from Link A and Link B are optically amplified by the optical amplifier as optical signals and are chromatic dispersion compensated by the dispersion compensation module. In addition, the accumulated chromatic dispersions of each WDM signal are measured individually by the chromatic dispersion monitor, which is connected with the output side of the optical amplification repeater via an optical coupler. The measured results are transferred to the control circuit of the variable chromatic dispersion compensator, to the rear stage of the dispersion compensation module which is disposed either before or after the single optical amplifier or between the stages of multistage optical amplifiers. This circuit controls the variable chromatic dispersion compensator for each WDM signals from Link A and Link B so as to allow the accumulated chromatic dispersion value to be that predetermined in system design, or so as to allow the received optical signal quality at the optical receiving end to be the highest in the control circuit.
As stated above, the WDM signals from Link A and Link B on which the accumulated chromatic dispersion compensation has been individually performed are input to the wavelength selective switch. The path shifting control is performed on the WDM signals as optical signals in the wavelength selective switch.
In the configuration example of the photonic network shown in FIG. 1, because the output port is only one port, both WDM signals from Link A and Link B are connected with Link C by the path shifting control in the wavelength selective switch. Both WDM signals from Link A and Link B are transmitted through Link C and are inputted to the optical receiving end. After the signals are optically amplified by the optical amplifier as optical signals and are chromatic dispersion compensated by the dispersion compensation module at the optical receiving end, the signals are branched into optical signals of each wavelength by the branching filter, and the signals are then received by the optical receiver by each wavelength. Here, a plurality of wavelength paths contained in the WDM signal which is connected with the optical receiving end via Link A, the HUB node, and Link C from the optical transmitting end A is called the wavelength path group (1), and a plurality of wavelength paths contained in the WDM signal which is connected with the optical receiving end via Link B, the HUB node, and Link C from the optical remitting end B is called the wavelength path group (2).
In the photonic network above, the accumulated chromatic dispersion compensation is performed independently for each Link, as stated earlier. Therefore, as shown in each of the accumulated chromatic dispersion vs. signal optical wavelength characteristics of Link A, B, C in FIG. 1, each Link may have different accumulated chromatic dispersion characteristics; in Link A, the wavelength dependency characteristics of the accumulated chromatic dispersion characteristics have a positive gradient to the wavelength axis, and in contrast, the wavelength dependency characteristics have a negative gradient to the wavelength axis in Link B. In addition, if the WDM signals of Link A and Link B which have differing accumulated chromatic dispersion characteristics are multiplexed in the HUB node and are input to Link C, the accumulated chromatic dispersion characteristics in the wavelength path group (1) and the wavelength path group (2) differ. As a result, the optimum accumulated chromatic dispersion compensation can not be performed on each wavelength path contained in both wavelength path group (1) and the wavelength path group (2) by using the dispersion compensation module contained in the single or multistage optical amplifiers in the Link C, and the dispersion compensation module and the variable dispersion compensator provided at the optical receiving end. Furthermore, due to the difference in the accumulated chromatic dispersion characteristics of each wavelength path, the distortions of transmitted waveforms differ with each wavelength path, and this waveform distortion significantly limits the transmission distance of the wavelength path.
In addition, in the WDM network connected in a mesh by a plurality of optical cross connects, a technology that relates to an optical network system which compensates the transmission characteristics deterioration attributable to the dispersion of the optical filter constituting the optical cross connect and its dispersion compensation control method thereof are disclosed (for example, see Reference 1).
Moreover, in the transoceanic wavelength division multiplexing (WDM) optical amplification transmission system which multiplexes and transmits a plurality of wavelengths, the references on the wavelength division multiplexing sea-bottom branching system that uses the WDM sea-bottom branching device which branches and inserts a certain wavelength are disclosed (for example, see Reference 2).    Reference 1: Japanese Patent Publication: Japanese Patent Laid-open Publication No. 2003-60577    Reference 2: Japanese Patent Publication: Japanese Patent Laid-open Publication No. 9-116494
As stated above, in the photonic network to which a conventional HUB node is applied, the accumulated chromatic dispersion compensation is performed independently for each Link between the optical receiving end and the HUB node, between the HUB nodes, and between the HUB node and the optical receiving end. Therefore, since the accumulated chromatic dispersion characteristics may differ with wavelength path, there arises a problem in that the transmission characteristics differ with wavelength path attributable thereto. This causes the transmission distance to differ with wavelength path and becomes a factor in the deterioration of system performance.